Surface micro-machining is a technique whereby freestanding and moveable structures are made on top of a substrate using thin film deposition and etching techniques. In this way, both the MEMS element, e.g. a resonator, oscillator, capacitor element, pressure sensitive element and so on, and its package can be processed on top of a substrate such as a silicon wafer. The MEMS element typically has a height of only several thin films measuring about 10 μm in total thickness. Furthermore, surface micro-machining allows for the definition of many thousands of MEMS elements onto a single wafer without making use of expensive assembly steps. This makes MEMS technology a particularly promising technology for miniaturization of functionality on an IC.
The MEMS element may be responsive to an actuator in the IC, e.g. in case of a capacitive MEMS, in which case the measured response of the MEMS element to the actuation signal is translated into a value of a parameter of interest. The MEMS element, which is typically suspended in a cavity over the substrate of the IC, is spatially separated from the actuator by a gap filled with a gas or vacuum, which gap forms a part of the cavity volume. In order to maximize the sensitivity of the MEMS element to the actuation signal, or to maximize the sensitivity of the IC to the detection of the displacement of the MEMS element, this gap should be kept as narrow as possible, as the electrostatic force applied by the actuator on the MEMS element typically scales with 1/W2, wherein W is the width of the gap expressed as the distance from the edge of the MEMS element facing the actuator or sensor and the edge of the actuator or sensor facing the MEMS element. For this reason, this gap often forms the narrowest clearance of the MEMS element from the cavity walls.
Such narrow clearances can cause problems in the manufacture of such MEMS elements. Many manufacturing processes rely on one or more wet etching steps to release the MEMS element, i.e. to form the cavity around the MEMS element, which can cause the MEMS element to stick against the cavity walls during subsequent drying steps. This phenomenon is commonly referred to as stiction, and renders the MEMS device non-functional. This problem can be addressed by critical point drying steps, but such techniques are relatively immature and costly.
Moreover, even if stiction problems can be avoided, it has been found by the present inventors that filament formation between the MEMS element and the cavity wall can still occur. An example of such filament formation is shown in FIG. 1, which depicts a scanning electron microscope image of a MEMS element 10 cleared from its surroundings by narrow trench 20 (the actuation gap) and a wider trench 30 providing electrical insulation from its surroundings. In the amplified section of the actuation gap, the formation of filaments bridging the gap can be clearly identified, as further indicated by the arrow.